The metro to the Westin looks direct, but it's a trap: at Montparnasse, the line 12 platforms are a solid 10-minute walk from the mainline train platforms, full of stairs and all.
If your suitcase is bulky, a taxi feels like money well spent, and getting one at Montparnasse is very easy if you pay close attention to signage - the taxi rank is well-organized and waiting is minimal. Traffic shouldn't be an issue in the early PM.
If it's just a 25-pound piece of cabin luggage, you can still probably manage the metro if you are really averse to taxis. I often have to go to Montparnasse station with a very similar load and a similar metro journey, and I typically take the metro there and take a taxi on the way back.
For Gare du Nord, without a direct metro, it's definitely taxi for me, but do be mindful of possible traffic delays.